1. Field of the Invention
The present invention relates generally to cryogenic liquid turbo-pumps, and more specifically to detecting cavitation within cryogenic liquid turbo-pumps.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
The success of a rocket launch depends upon the reliability of the launch vehicle, with the riskiest part of the vehicle being the rocket engine. For launch vehicles fueled by a pump fed liquid propellants, the highest risk component is the engine's turbo-machinery. The turbo-pump assembly (TPA) accounted for 32% of the launch failures from the 1,442 US launches during the years 1965 to 2004. Moreover, of the 4,400 launches worldwide, 37% of the engine failures were due to the turbo-pump assembly.
The top two life issues for turbo-pumps are the turbine blades and the bearings. The third most common failure in turbo-pumps is due to cavitation. The original Rocketdyne turbo-pumps on the Space Shuttle Main Engine (SSME) had to be replaced every one to three missions due to insufficient life in either the turbine blades or the bearings. Their design life goal was for 55 missions.
Currently in turbo-pumps, there is no direct measurement of turbine blade health. In the gas turbine industry, a gas path analysis is typically done, with information on the expected performance of the turbine coming from either a component map (which is input to the generalized cycle program) or a component level model (part of a cycle deck specific to the engine type). The actual component performance is determined by comparing the measurements, such as exhaust temperature, with their expected values by calculating error terms. The solver iterates on these error terms to some tolerance, thus computing the actual component performance. The actual measurements and actual performance can then be compared to a historical database and/or further processed by specific algorithms to determine the turbine's health.
Similarly, the existing methods for rolling element bearing health monitoring rely on indirect measurements, such as accelerometers and acoustic emission probes for vibratory data, and thermocouples on the bearing outer ring. These measurements try to capture the most typical rolling element bearing failure modes: contact fatigue and skidding damage. These failures increase heat generation due to friction resulting from the fault. However, 90% of the heat generation occurs on the bearing inner raceway, so the outer ring thermocouple is often the last indicator of a fault. Additionally, for accelerometers and acoustic emission probes to detect a fault, the vibration signal has to be strong enough to pass through the bearing dead band clearance. Also, thus far, due to their sensitivity, acoustic emission probes have not been actively used for in-flight health management decisions. These are all limitations of the present technology.
One of the main limiting factors in the performance of a pump is due to cavitation. Cavitation is the phenomenon where the local pressure of a liquid drops below the vapor pressure at the given temperature, causing a vapor bubble to form. This happens in pumps in regions where the flow is accelerated so quickly that the local static pressure drops below vapor pressure. Cavitation usually occurs just downstream of the pump inlet, before the pressure has risen sufficiently high enough above vapor pressure. In a turbo-pump, the onset of cavitation typically occurs on the inducer blade tips, in the blade tip vortices.
Cavitation causes a break down in the suction performance of a pump, and can also be highly damaging to the pump blades. When the bubbles are swept along to regions of higher pressure, the metal surface near where the bubbles collapse will experience erosion. Lastly, cavitation affects the flow field through a pump and can cause unsteadiness. This unsteadiness can manifest itself in a manner similar to rotating stall in a compressor or to a compressor surge.
Raising the inlet pump pressure increases the margin against cavitation. This margin is called net positive suction head, or NPSH. However, there exists many applications where the inlet pressure cannot be raised, such as in a rocket engine turbo-pump. The lower the tank pressure is, the less structural reinforcement is needed for the propellant tank, and hence decreasing the weight of the flight vehicle. Most turbo-pumps employ advanced inducer designs in order to avoid cavitation in the main pump. Additionally, depending on if the propellant is cryogenic, thermodynamic suppression head (TSH) effects may have to be taken into account. The TSH effects cause a local temperature drop in the fluid which acts to suppress cavitation because the temperature drop also reduces the vapor pressure. TSH effects vary depending on the fluid. For liquid hydrogen, it is often a considerable benefit, adding up to 100+ ft of NPSH margin. In liquid oxygen, the benefit is less significant.
At this time, there is not a reliable way to predict the inception of cavitation. What is currently done to test a design for its resistance to cavitation is to incrementally drop the inlet pressure while the pump head rise is measured. The collapse of the bubbles is very noisy and hence once caviation has began, it can be picked up by acoustic probes. As shown in FIG. 1, the primary amount of blade damage would occur well before the measured performance breakdown of the pump. The Thoma cavitation number shown on the x-axis is simply a measure of the difference between the inlet and vapor pressures, divided by the total pressure rise.
U.S. Pat. No. 7,231,817 B2 issued to Smed et al. on Jun. 19, 2007 and entitled INSPECTION SYSTEM FOR A TURBINE BLADE REGION OF A TURBINE ENGINE discloses an IR camera used to image hot sections of the turbine blades to determine the engine health. The inspection system includes a viewing tube that extends through the engine casing to a location in which the IR viewing will take place, in this case at a location pointing right at the turbine blades. The system also includes one or more lens and an optical lens within the tube, and a cap on the blade end with an aperture adjacent to the cap to allow light to pass through. The Smed U.S. Pat. No. 7,231,817 B2 is incorporated herein by reference.